1. Field of the Invention
The present invention relates to a method and apparatus for forming a thin film on a surface of a substrate such as a glass substrate, a plastic substrate, or the like, the thin film comprising any of various materials such as metallic materials, semiconductor materials, insulating materials, and the like.
2. Description of the Related Art
A conventional technique widely known as a technique for forming a thin film on a surface of a substrate uses a sputtering phenomenon. This thin film forming technique is capable of forming films ranging from crystal films to amorphous films using almost all materials including metals, semiconductors, and insulators, and forming films of single elements, compounds, alloys, and the like. Thin film forming technique is also excellent in productivity, and thus widely used in various fields such as semiconductors, displays, surface processing, and the like.
In principle, sputtering is performed on the assumption that plasma is generated, and thus charged particles with relatively high energy, such as ions, electrons, and the like, and high-energy particles such as sputtered atoms are generated. By positively using such high energy, films with high mechanical strength such as adhesive force, and various functional thin films have been formed and brought into practical use. However, as studies progressed, it has become clear that in order to further improve the characteristics of a semiconductor such as GaAs, a functional material such as a high-temperature superconductor, for example, yttrium oxide, bismuth oxide, or the like, or an organic fluorescent material for organic EL, the energy of particles must be precisely controlled to remove particles having excessively high energy, thereby suppressing the adverse effect on film surfaces. On the other hand, a substrate on which a thin film is formed is exposed to high-energy particles and reaches a high temperature. Therefore, inorganic substrate are frequently used, and the application of sputtering to organic substrates for forming organic films, which easily change in quality and deteriorate due to high-energy particles, is limited to narrow ranges. For example, in multilayer wiring of a semiconductor device, only polyimide having relatively high durability is currently put into practical use for organic interlayer insulating films from the viewpoint described above.
Under these conditions, there have been active studies for controlling the energy of particles to further improve the performance of functional thin films and to extend the application of the sputtering technique to organic substrates. Examples of proposed methods include a method of applying a bias to a substrate to control the energy of charged particles, an off-axis sputtering method in which a substrate is separated from a sputtering space, and a method of optimizing the magnetic field strength of a sputtering cathode magnet to decrease a discharge voltage. Furthermore, methods capable of forming the above-described organic thin films include the following:
(1) The rate of occurrence of high-speed electrons and the energy of particles flying to a substrate are decreased by a DC sputtering method using Kr or Xe gas as a sputtering gas at an operating pressure of 10 Pa or more (Japanese Unexamined Patent Application Publication No. 10-12572).
(2) A coil made of the same material as a target material is disposed near a substrate, and an AC voltage is applied for converging electrons and ions in the vicinity of a surface of the target and for preventing incidence on the substrate (Japanese Unexamined Patent Application Publication No. 10-140344).
(3) A thin film forming method and apparatus using a transport sputtering process in which a substrate and a hollow target are disposed in a vacuum chamber, a high-speed sputtering gas is introduced from an end of the target to which a voltage is applied for generating a plasma, and the sputtered particles produced in the hollow portion of the target are transported by the sputtering gas and deposited on the substrate. Furthermore, an exhaust port is provided vertically to the central axis of the target, for exhausting the sputtering gas containing the sputtered particles. The substrate is disposed near the exhaust port apart from the central axis or within the exhaust port so that a thin film is formed while preventing the substrate from being influenced by plasma, high-energy particles, electromagnetic waves, and the like (Japanese Unexamined Patent Application Publication No. 2001-140066).
In the method and apparatus disclosed in each of the above documents, conditions for sufficiently removing high-energy particles such as electrons, ions and neutral particles, and electromagnetic waves emitted from the plasma, such as ultraviolet rays, are basically contrary to conditions for achieving mass productivity and increasing the area of a substrate surface on which a thin film formed. Therefore, it is very difficult to simultaneously satisfy these conditions. This problem is common to the sputtering method and a physical deposition method other than the sputtering method, for example, an evaporation method, to varying degrees.
Namely, in the existing evaporation method and sputtering method, evaporation, vaporization, or scattering of a film-forming material is isotropic and has no directivity. Thus, in order to avoid the influence of the high-energy particles and electromagnetic waves on the substrate surface during film deposition, an absolute distance or a blocking wall must be interposed between the substrate and a portion where the film-forming material is evaporated, vaporized, or scattered. As a result, the deposition rate is inevitably decreased.
Furthermore, in a reactor, many particulates adhere to a portion (for example, the inner wall of the reactor) other than the substrate. Therefore, the energy efficiency of the film-forming material is decreased, and the frequency of maintenance (cleaning) of the apparatus is increased, thereby causing difficulty in improving the working ratio.
The method disclosed in Japanese Unexamined Patent Application Publication No. 2001-140066 is the transport sputtering method in which the sputtered particles are transported from the sputtering region to the substrate using the high-speed gas and the hollow target, so that high-energy charged particles and neutral particles can be effectively removed before reaching the substrate. However, in order to actually optimize the arrangement of the target and the substrate, the flow rate of the transport gas, and the operating pressure, a flow rate of at least 50 m/s is required for forming a viscous flow for transporting a substance under a pressure (100 Pa or less) which allows sputtering due to the occurrence of the stable plasma. Since a giant exhaust system is required for securing such a high flow rate, practical application is difficult.
A thin film forming method other than the evaporation method and the sputtering method is a plasma chemical vapor deposition (CVD) method in which in principle, high-energy particles are not involved. However, in this CVD method, usable source material gases are greatly limited, and only amorphous Si, and oxide such as silica, titania, and the like are currently put into practical use. Although, in principle, a metal film can be formed using an organometallic vapor as a source material gas, many organometallic vapors have high toxicity and are hard to handle. Also, excessive investment is required for equipment for supplying and removing gases, and thus a CVD apparatus cannot be replaced by an existing evaporation apparatus or sputtering apparatus in the industrial field except for some extraordinary reason.